The present invention relates generally to digital data receivers and particularly concerns an AGC system for a digital television signal receiver.
A number of systems have recently been proposed for transmitting and receiving television signals in digital form. The television signal may comprise, for example, a compressed wide band HDTV signal or one or more compressed NTSC signals. The two most widely promoted modulation techniques for effecting such transmission are quadrature amplitude modulation (QAM) and vestigial side band modulation (VSB). U.S. Pat. No. 5,087,975 discloses a VSB system for transmitting a television signal in the form of successive M level symbols over a standard 6 MHz television channel, with a relatively small, that is low power level, pilot at the lower edge of the channel. While the number of levels M, (the VSB mode that characterizes the symbol levels) may vary, the symbol rate is preferably fixed, such as at a rate of 684H (about 10.76 megasymbols per second) where H is the NTSC horizontal scanning frequency. The number of symbol levels used in any particular arrangement is largely a function of the signal-to-noise ratio that characterizes the transmission medium with a smaller number of symbol levels being used in situations where the signal-to-noise ratio is low, i.e. in noisy environments. For a cable system that provides a relatively benign environment, a system accommodating symbol levels of 16, 8, 4 and 2 provides flexibility to satisfy most conditions. For terrestrial or over the air broadcasts where the environment is less benign, a VSB mode of 8 (i.e. 8-level data symbols) is appropriate. It will be appreciated that lower values of M can provide improved signal-to-noise performance at the expense of a reduced transmission bit rate. For example, assuming a rate of 10.76 megasymbols per second, a two level VSB signal (one bit per symbol) provides a transmission bit rate of 10.76 megabits per second whereas a four level VSB signal (two bits per symbol) provides a transmission bit rate of 21.52 megabits per second.
Proper operation of a digital receiver, such as a television receiver, requires the received carrier signal to be acquired relatively rapidly and the gain of the RF and IF sections of the receiver to be appropriately adjusted. Carrier acquisition in QAM receivers is relatively difficult because of the absence of any sort of pilot. While the use of a pilot in the above-mentioned VSB system greatly facilitates carrier acquisition, difficulties are nevertheless encountered due to the relatively low level of the pilot and the limited pull in range of synchronous demodulators that are used in these receivers. The above-mentioned copending related applications are concerned with enhancing the pull in of the FPLL and dealing with the biphase stability of the frequency and phase locked loop in the synchronous demodulator. This invention is directed to a novel AGC system for use in a digital television receiver where the signal includes a pilot.